There is an increasing demand for higher quality premium coke for the manufacture of large graphite electrodes for use in electric arc furnaces employed in the steel industry. The quality of premium coke used in graphite electrodes is often measured by its coefficient of thermal expansion, which can vary from as low as -5 to as high as +8 centimeters per centimeter per degrees centigrade times 10.sup.-7. Users of premium coke continuously seek graphite materials having lower CTE values. Even a small change in CTE can have a substantial effect on large electrode properties. Graphite electrode density is also of importance in characterizing the quality of graphite electrodes. Usually the higher the density, the better the electrode.
Premium coke is manufactured by delayed coking in which heavy hydrocarbon feedstocks are converted to coke and lighter hydrocarbon products. In the process the heavy hydrocarbon feedstock is heated rapidly to cracking temperatures and is fed continuously into a coke drum. The heated feed soaks in the drum and its contained heat which is sufficient to convert it to coke and cracked vapors. The cracked vapors are taken overhead and fractionated with the fractionator bottoms being recycled to the feed if desired. The coke accumulates in the drum until the drum is filled with coke at which time the heated feed is diverted to another coke drum while the coke is removed from the filled drum. After removal, the coke is calcined at elevated temperatures to remove volatile materials and to increase the carbon to hydrogen ratio of the coke.
In the manufacture of large graphite electrodes, calcined premium coke particles obtained from the delayed coking process are mixed with pitch and then baked at elevated temperatures to carbonize the pitch.
According to this invention, premium coke having improved properties is obtained by subjecting the coke to a heat soak which is carried out at substantially the same temperature as that used in the coking operation. Coke properties are also improved by coking at lower temperatures than those normally used in the coking operation.